Traffic control devices, such as signal lamps, play a major role in enabling the existence of modern traffic systems. As such, they also account for high costs to metropolitan and other political jurisdictions that must procure, install, maintain, and replace such signal lamps.
Insufficient light output, flexibility in accepting various power sources, overheating, and susceptibility to damage or degradation due to short- or long-term subjection to transient power surges are just some of the issues that have been persistent problems in the field of signal lamps.
Traffic control devices, such as left turn signals and other traffic signs, serve the well-known function of directing traffic. To be effective, such signs must be easily visible from significant distances. However, one drawback of conventional traffic signs is that they have a permanent and unchanging nature. For example, the only way that a conventional “no right turn” traffic sign can prohibit right turns during the hours of 7:00 AM and 7:00 PM is to have that qualification inscribed on the sign itself.
Inscribing such qualifications is fraught with two great limitations. First, a traffic sign typically has a severely limited area within which to inscribe such a qualification. Moreover, in order to be effective, the inscribed qualification must be easily visible from significant distances. Therefore, the inscribed qualification must be typeset using large letters, which even further limits potential content.
Second, the inscribed qualification is typically affixed to the sign in a relatively permanent manner. Consequently, the inscribed qualification cannot be easily changed on frequent basis.
Some attempts to solve this problem have been made by implementing light based signs. Such a sign can be switched on during active time periods, and otherwise switched off. However, such signs have encountered numerous problems, such as overheating, insufficient visibility, over-brightness in darkness, and unreliability.
Thus, what is needed is a traffic sign that can overcome those and other problems while proving traffic control during selected time periods without resorting to inscribing of qualifications.
Light emitting diode signal lamps produce light output using light emitting diodes. Such diodes are traditionally organized in an array. FIG. 30 schematically shows a side view of a signal lamp that includes light emitting diodes (LEDs) 12 arranged in a uniformly distributed array. Diffuser 14 is oriented in relation to LEDs 12 so as to cosmetically enhance the appearance of the signal lamp of FIG. 30. The diffuser 14 prevents viewers from clearly seeing individual LEDs and, more importantly, individual LED failures. The signal lamp of FIG. 30 also includes a collector lens 16, which focuses light received from the diffuser in a centering fashion in order to meet requirements of a typical governmental traffic lamp specification. The special lenses required by the signal lamp of FIG. 30 increase its cost, and other problems will also be apparent to those skilled in the art.
Turning to FIG. 31, another signal lamp of the prior art is schematically depicted, including LEDs 18 arranged in a densely configured square-shaped array in the center of the signal lamp. The motivation for configuring LEDs 18 in a dense square-shaped array in the center of the signal lamp is to achieve compliance with a governmental specification that regulates traffic lamps. Fresnel lens 20 is oriented relative to the LEDs 18 in order to cosmetically improve the light output distribution of the signal lamp of FIG. 31 by somewhat spreading light output away from the center of the signal lamp, while leaving the center very bright. However, among the drawbacks of the signal lamp of FIG. 31 is that a special lens is required, thereby increasing cost of the system. Furthermore, the LED signal lamp of FIG. 31 uses a relatively small number of LEDs, and thus could be subject to a corresponding reduction in reliability.
A third type of signal lamp of the prior art is shown schematically in FIG. 32. LEDs 22 are arranged in a uniformly distributed array, being adapted to produce undiffused light 24. A diffuser 25 is oriented relative to the LEDs 22 for intercepting and converting at least some of the undiffused light 24 in order to produce diffused light 26. However, the signal lamp of FIG. 32, like those of FIGS. 1 and 2, requires a special lens, thereby increasing cost of the system.
Another problem that commonly occurs in the field of light emitting diode signal lamps is that when one or more light emitting diodes fail, the surviving light emitting diodes suffer accelerated aging as a direct result. FIG. 35 depicts a string of light emitting diode stages, including a first stage 48 and a second stage 50. Each stage includes LEDs 52. When constant voltage is maintained across the string, the failure of one of the LEDs 52 within the first stage 48 will cause the surviving LEDs 52 of the first stage 48 to suffer accelerated degradation due to the correspondingly higher current load they will be forced to carry.
Traffic signal lamps for an intersection are typically connected to a conflict monitor in order to detect the occurrence of conflicting states among traffic signals; for example, all traffic signals green. Upon detection of such a problem, the conflict monitor will cause the lights of the intersection to enter a default “safe state”; for example, one set of opposing lights flashing yellow, the other set of opposing lights flashing red.
The conflict monitor can also send the traffic signals of the intersection into a safe state if all of the traffic signals facing a given direction fail. FIG. 38 shows a prior art schematic representation of an incandescent signal lamp 80 of the prior art connected to a conflict monitor of the prior art 82. Upon failure, incandescent signal lamp 80 no longer passes current, which is detected by the conflict monitor 82. Thus the conflict monitor 82 can engage appropriate logic to manage the signal lamps of the intersection in response to detection of failed signal lamps. FIG. 39 shows conflict monitor 82 connected to a light emitting diode signal lamp 84 of the prior art. A problem of prior art light emitting diode signal lamps is that they pass current even after having failed. As a result, the conflict monitor is not aware of such failures, and is hindered in taking appropriate action in response to failure of light emitting diode signal lamps.
The LEDs used in light emitting diode signal lamps dim with age. Once such LEDs have dimmed to the point that their light output falls below a desired level, they should be replaced. In addition, some will fail before dimming sufficiently to require replacement. Such failures not only have an immediately negative impact on the light output of the signal lamp, they can also result in the above-described accelerated degradation of the surviving LEDs. One response in the industry has been to replace every LED signal lamp after a fixed amount of time, such as 3 years, whether a particular lamp needs to be replaced or not. However, such a blind replacement program does not adequately address signal lamps that fail prior to their scheduled replacement or signal lamps that would have significant useful life beyond their scheduled replacement. In the former event, a dangerous situation could result from failure of an in-service signal lamp. In the latter event, unnecessary costs are directly incurred.